1. Field
The presently disclosed embodiments relate generally to the efficient power management and control of arrays of light emitting devices such as Cold Cathode Fluorescent Lamps and Light Emitting Diodes. More specifically, the disclosed embodiments relate to improving the power efficiency in the backlighting of Liquid Crystal Displays.
2. Background
Arrays of Cold Cathode Fluorescent Lamps (CCFLs) are now commonly used for backlighting Liquid Crystal Displays (LCDs) in notebook and laptop computer monitors, car navigation displays, point of sale terminals and medical equipment. CCFLs have quickly been adopted for use as the backlight in notebook computers, and various portable electronic devices because it provides superior illumination and cost efficiency. Many of these applications allow only a very limited supply of power, and thus, the power consumption must be judiciously managed. Current methods of managing power in LCDs are inefficient. As display panels increase in size, it is necessary for more power efficient designs and methods to be deployed.
Typically, a high voltage DC/AC inverter is required to drive the CCFL because the lamp uses a high Alternating Current (AC) operating voltage. With the increasing size of the LCD panel, multiple lamps or arrays are required to provide the necessary illumination. Therefore, an effective inverter is required to drive the CCFL arrays.
Intensity of illumination is determined primarily by the operating current applied to each CCFL by a converter. In conventional lamp panel arrays, each lamp is driven by its own inverter which is typically 70% efficient in delivering power to the lamp. This efficiency, or lack thereof, coupled with the redundant inverter circuitry necessary for powering a multiple lamp array results in a significant amount of power consumption.
It would be desirable to conserve power by using a single inverter to power an entire lamp panel array. However, using a shared inverter forces the operating current of all the lamps to the same current as determined by a reference lamp. Since each lamp has characteristics that cause it to vary in brightness and intensity due to age, replacement and inherent manufacturing variations, applying the same reference voltage to each lamp, without adjusting for individual lamp variations, results in a different intensity of illumination between lamps in the array. Such varying illumination intensities cause undesirable lines to be visible in the LCD display. The prior art attempts to solve this problem by adding a thicker diffuser panel between the lamps and the LCD display to smooth out diffuse lighting differences. This in turn results in more power consumption by the lamps in order to obtain the optimum light intensity at the surface of the display. Thus, reducing the number of inverters and redundant circuitry to conserve power requires a new design and method which effectively removes variations which cause unacceptable varying lamp intensities while minimizing total power consumption.
CCFL component variations can be broken down into two classifications, “fixed” and “dynamic.” Fixed variations are defined as inherent manufacturing variations between lamps and their inverter circuit components, such as the transformer and primary drive circuit. Conventional lamp designs utilizing a single or small number of CCFLs attempt to minimize fixed variations by manually adjusting a potentiometer in each lamp circuit. This method is impractical in larger multiple lamp arrays commonly found today. Instead, these fixed variations in components are minimized by forcing the inverter circuit to supply more power than is necessary in order to compensate for a more demanding lamp in the array, or for a “weaker” transformer or primary drive circuit. This “overcompensating” to account for variations is another inefficient use of power since the extra power may not be required at all or to the degree compensated. Accordingly, there is a need for a panel display design and method which automatically establishes an optimal lamp power for each lamp in the array despite fixed variations in components in singular inverter applications.
Dynamic variations are variations in lamp luminance over use, time and temperature, as well as variations in DC supply voltage (battery) to the inverter circuit. The prior art solution described above is not applicable since a potentiometer set point is adjusted only at manufacture and remains fixed over time. Again, conventional designs attempt to get around this problem by using an initial current setting that is higher in value than what is actually required for optimal lamp luminance. This results in the inefficient use of power. Accordingly, there is a need for an inverter circuit design which automatically establishes and delivers an optimal power to each lamp in light of changing component variations such as aging, temperature, and battery or other DC supply voltage, rather than relying on overcompensation of power to ensure each lamp is properly driven.
A known method of adjusting lamp luminance is incorporating dimming capability into the circuit design. Dimming is typically accomplished through either current limiting or PWM. In the current limiting mode, the lamp current is reduced, but the lamp stays on all the time. In the PWM mode, the lamp is turned full on and off at a repetition rate of about 100 to 400 Hz with a dimming range being determined by the duty cycle (fraction on time). For example, if a lamp is dimmed to 75% luminance, a PWM waveform with a duty cycle of 75% is used. Each time the lamps are turned on, there is a need to re-apply a “strike voltage.” The strike voltage corresponds to twice the operating voltage and is applied until the lamp switches to its fluorescent state, whereupon the voltage can be reduced to a “sustaining voltage.” Conventional analog controllers detect when a strike has occurred by utilizing a current control loop having a frequency response of about a few hundred (200) Hz. This slow response time delays the reduction in lamp voltage from the strike value to the sustaining value resulting in inefficient use of power. Accordingly, there is a need for a panel display design and method which utilizes a current control loop which has a faster response time (such as 20M Hz) for detecting fluorescence and reducing lamp voltage to a sustaining voltage.
Another inefficient use of power in large displays with multiple lamps driven by a single inverter is the power switching control circuit which sets total lamp currents. In conventional applications where a single inverter is used to drive a lamp array, power to the lamp is switched with a Pulse Width Modulator (PWM). The primary circuit of the inverter contains Mosfets which control power to the transformer. The time it takes to switch the Mosfets between their on and off states results in high circuit resistance which in turn consumes significant amounts of power. In conventional analog controller circuits, the timing of the primary circuit can only be improved by changing out the associated passive components surrounding the analog controller. Accordingly, it is desired that the timing of the primary circuit be more easily adapted to manufacturing variations and changing conditions through automatic calibration of the timing of pulses used to drive the Mosfets, thereby minimizing power consumption.
As the market place has driven down the cost of CCFLs, resulting in widespread use of multiple lamp array panels, the demand for power efficiency has increased. Conventional types of backlights for LCD devices are not fully satisfactory with respect to the amount of power they consume. Thus, there is a need in the art for a display panel design and method capable of individually sensing and adjusting the current applied to an array of CCFLs in multiple lamp LCD displays while also reducing the amount of power consumption.